Previous studies from this laboratory have demonstrated that the structural gene for the muscle enzyme, phosphorylase kinase, and for the genes that control the resting concentration of glycogen are on the X chromosome. Our future studies will be a continuing effort to determine the precise mechanisms that control the glycogen concentration in resting muscle, and to determine what enzymes are involved. Presumably all of these enzymes would have either their structural genes or a controlling regulatory gene on the X chromosome. The primary laboratory tool in this effort will be the I strain mouse. These animals contain only traces of phosphorylase kinase in skeletal muscle and yet are capable of glycogen degradation in response to electrical or epinephrine stimulus. The mechanism of activation of phosphorylase b in these animals is still unknown, but our recent findings suggest that phosphorylase in these mice is more sensitive to changes in pH. A more precise definition of this statement and the ramifications of such changes will be the subjects of our future investigations.